SIST EN 61094-6:2005

Measurement microphones -- Part 6: Electrostatic actuators for determination of frequency response

This part of EN 61094 - gives guidelines for the design of actuators for microphones equipped with electrically conductive diaphragms; - gives methods for the validation of electrostatic actuators; - gives a method for determining the electrostatic actuator response of a microphone. The applications of electrostatic actuators are not fully described within this standard but may include - a technique for detecting changes in the frequency response of a microphone, - a technique for determining the environmental influence on the response of a microphone, - a technique for determining the free field or pressure response of a microphone without specific acoustical test facilities, by the application of predetermined correction values specific to the microphone model and actuator used, - a technique applicable at high frequencies not typically covered by calibration methods using sound excitation.

Messmikrofone -- Teil 6: Elektrostatische Anregeelektroden zur Ermittlung des Frequenzgangs

Microphones de mesure -- Partie 6: Grilles d'entraînement pour la détermination de la réponse en fréquence

La présente partie de la EN 61094 - donne les lignes directrices pour la réalisation de grilles d'entraînement destinées aux microphones possédant une membrane faite dans un matériau électriquement conducteur; - donne des méthodes pour la validation des grilles d'entraînement; - donne une méthode pour déterminer la réponse d'un microphone à la grille d'entraînement. Les applications des grilles d'entraînement ne sont pas complètement décrites dans cette norme mais peuvent comprendre - une technique pour détecter les variations de la réponse en fréquence d'un microphone, - une technique pour déterminer l'influence des conditions ambiantes sur la réponse d'un microphone, - une technique de détermination de la réponse en fréquence en champ libre ou en pression d'un microphone sans nécessiter un équipement acoustique spécifique, par application de valeurs de corrections prédéterminées spécifiques des modèles de grille d'entraînement et de microphone utilisés, - une technique d'étalonnage applicable aux fréquences élevées non couvertes spécifiquement par des méthodes d'étalonnage utilisant une excitation acoustique.

Meritev mikrofonov – 6. del: Elektrostatični aktuatorji za ugotavljanje frekvenčnega odziva (IEC 61094-6:2004)

General Information

• Status: Published
• Publication Date: 30-Nov-2005
• ICS: 17.140.50 - Electroacoustics
• ICS: 33.160.50 - Accessories
• Technical Committee: SS EIT - Technical Board for the electrotechnical, information technology and telecommunications fields
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-Dec-2005
• Due Date: 01-Dec-2005
• Completion Date: 01-Dec-2005
• Ref Project: EN 61094-6:2005 - Measurement microphones - Part 6: Electrostatic actuators for determination of frequency response

Standards Content (Sample)

SLOVENSKI SIST EN 61094-6:2005

STANDARD
december 2005
Meritev mikrofonov – 6. del: Elektrostatični aktuatorji za ugotavljanje
frekvenčnega odziva (IEC 61094-6:2004)
Measurement microphones – Part 6: Electrostatic actuators for determination of
frequency response (IEC 61094-6:2004)
ICS 17.140.50; 33.160.50 Referenčna številka
SIST EN 61094-6:2005(en)
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

---------------------- Page: 1 ----------------------

EUROPEAN STANDARD EN 61094-6
NORME EUROPÉENNE
EUROPÄISCHE NORM January 2005

ICS 17.140.50


English version


Measurement microphones
Part 6: Electrostatic actuators for determination of frequency response
(IEC 61094-6:2004)


Microphones de mesure Messmikrofone
Partie 6: Grilles d'entraînement Teil 6: Elektrostatische Anregeelektroden
pour la détermination zur Ermittlung des Frequenzgangs
de la réponse en fréquence (IEC 61094-6:2004)
(CEI 61094-6:2004)






This European Standard was approved by CENELEC on 2004-12-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and
notified to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden,
Switzerland and United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels


© 2005 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 61094-6:2005 E

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EN 61094-6:2005 - 2 -
Foreword
The text of document 29/562/FDIS, future edition 1 of IEC 61094-6, prepared by IEC TC 29,
Electroacoustics, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC
as EN 61094-6 on 2004-12-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2005-09-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2007-12-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61094-6:2004 was approved by CENELEC as a European
Standard without any modification.

---------------------- Page: 3 ----------------------

- 3 - EN 61094-6:2005
Annex ZA
(normative)

Normative references to international publications
with their corresponding European publications
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
NOTE Where an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
1) 2)
IEC 61094-1 - Measurement microphones EN 61094-1 2000
Part 1: Specifications for laboratory
standard microphones

1) 2)
IEC 61094-2 - Part 2: Primary method for pressure EN 61094-2 1993
calibration of laboratory standard
microphones by the reciprocity technique

1) 2)
IEC 61094-3 - Part 3: Primary method for free-field EN 61094-3 1995
calibration of laboratory standard
microphones by the reciprocity technique

1) 2)
IEC 61094-5 - Part 5: Methods for pressure calibration EN 61094-5 2001
of working standard microphones by
comparison

ISO/IEC Guide 1995 Guide to the expression of uncertainty in - -
Express measurement (GUM)





1)
Undated reference.
2)
Valid edition at date of issue.

---------------------- Page: 4 ----------------------

NORME CEI
INTERNATIONALE
IEC



61094-6
INTERNATIONAL


Première édition
STANDARD

First edition

2004-11


Microphones de mesure –
Partie 6:
Grilles d'entraînement pour la détermination
de la réponse en fréquence

Measurement microphones –
Part 6:
Electrostatic actuators for determination
of frequency response

 IEC 2004 Droits de reproduction réservés  Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in any
utilisée sous quelque forme que ce soit et par aucun procédé, form or by any means, electronic or mechanical, including
électronique ou mécanique, y compris la photocopie et les photocopying and microfilm, without permission in writing from
microfilms, sans l'accord écrit de l'éditeur. the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: [phone] Telefax: [phone] E-mail: [e-mail] Web: www.iec.ch
CODE PRIX
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Pour prix, voir catalogue en vigueur
For price, see current catalogue

---------------------- Page: 5 ----------------------

61094-6  IEC:2004 – 3 –
CONTENTS
FOREWORD.7

1 Scope.11
2 Normative references .11
3 Terms and definitions .13
4 Reference environmental conditions .13
5 Principle of electrostatic actuator operation .13
5.1 General .13
5.2 Electrostatic pressure.15
5.3 Electrostatic actuator response .21
6 Actuator design .23
6.1 General .23
6.2 Design.23
7 Validation .25
7.1 General .25
7.2 Repeatability of measurements .25
7.3 Uniformity of actuators of a given model.25
7.4 Uniformity of the difference between actuator and pressure response levels .25
8 Measurement of electrostatic actuator response.27
8.1 System for measurement of electrostatic actuator response .27
8.2 Uncertainty components .29
9 Applications of an electrostatic actuator .33
9.1 General .33
9.2 Verification of the frequency response of a measurement system.33
9.3 Determination of the environmental characteristics of microphone
measurement systems.33
9.4 Determination of free-field and pressure frequency responses.35
9.5 Measurement of actuator response at very high frequencies .35

Annex A (informative) Examples of electrostatic actuator designs.37
Annex B (informative) Set-up for measuring electrostatic actuator response .43
Annex C (informative) Typical uncertainty analysis .45
Annex D (informative) Difference between free-field-, pressure- and actuator
responses for typical models of measurement microphones.51

Figure 1 – Principle of microphone and electrostatic actuator .17
Figure 2 – Lumped parameter model of a measurement microphone excited by an
electrostatic actuator .21
Figure A.1 – Example of electrostatic actuator for type WS1 microphones .37
Figure A.2 – Example of an electrostatic actuator for type WS2 microphones .39
Figure A.3 – Examples of electrostatic actuators forming integral parts of the
microphone protection grids.41
Figure A.4 – Example of an electrostatic actuator combined with  weather-resistant
protection .41

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61094-6  IEC:2004 – 5 –
Figure B.1 – Typical set-up for measuring the electrostatic actuator response of a
microphone.43
Figure D.1 – Examples of differences between relative pressure and actuator
frequency responses for four different type of measurement microphone: WS1P (a),
WS1F (b) of nominal sensitivities –26 dB re 1V/Pa and WS2P (c) and WS2F (d) of
nominal sensitivities –38 dB re 1V/Pa .51
Figure D.2 – Examples of differences between relative free-field and actuator
frequency responses for type WS1, WS2 and WS3 microphones when used without
protection grids.51
Figure D.3 – Example of model dependent difference between relative free field and
actuator frequency responses for a type WS2 microphone when used with its
protection grid.53
Figure D.4 – Example on the determination of a relative free-field frequency response b)
by adding the model dependent free-field to actuator difference as shown in Figure D.3
to the electrostatic actuator response of a microphone a) .53

Table C.1 – Uncertainties .49

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61094-6  IEC:2004 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

MEASUREMENT MICROPHONES –

Part 6: Electrostatic actuators for determination
of frequency response


FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61094-6 has been prepared by IEC technical committee 29:
Electroacoustics.
The text of this standard is based on the following documents:
FDIS Report on voting
29/562/FDIS 29/565/RVD

Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

---------------------- Page: 8 ----------------------

61094-6  IEC:2004 – 9 –
IEC 61094 consists of the following parts, under the general title Measurement microphones:
Part 1: Specifications for laboratory standard microphones
Part 2: Primary method for pressure calibration of laboratory standard microphones by the
reciprocity technique
Part 3: Primary method for free-field calibration of laboratory standard microphones by the
reciprocity technique
Part 4: Specifications for working standard microphones
Part 5: Methods for pressure calibration of working standard microphones by comparison
Part 6: Electrostatic actuators for determination of frequency response
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.

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61094-6  IEC:2004 – 11 –
MEASUREMENT MICROPHONES –

Part 6: Electrostatic actuators for determination
of frequency response



1 Scope
This part of IEC 61094
– gives guidelines for the design of actuators for microphones equipped with electrically
conductive diaphragms;
– gives methods for the validation of electrostatic actuators;
– gives a method for determining the electrostatic actuator response of a microphone.
The applications of electrostatic actuators are not fully described within this standard but may
include
– a technique for detecting changes in the frequency response of a microphone,
– a technique for determining the environmental influence on the response of a microphone,
– a technique for determining the free-field or pressure response of a microphone without
specific acoustical test facilities, by the application of predetermined correction values
specific to the microphone model and actuator used,
– a technique applicable at high frequencies not typically covered by calibration methods
using sound excitation.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 61094-1, Measurement microphones – Part 1: Specifications for laboratory standard
microphones
IEC 61094-2, Measurement microphones – Part 2: Primary method for pressure calibration of
laboratory standard microphones by the reciprocity technique
IEC 61094-3, Measurement microphones – Part 3: Primary method for free-field calibration of
laboratory standard microphones by the reciprocity technique
IEC 61094-5, Measurement microphones – Part 5: Methods for pressure calibration of working
standard microphones by comparison
ISO/IEC GUIDE EXPRESS: 1995, Guide to the expression of uncertainty in measurement
(GUM)

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61094-6  IEC:2004 – 13 –
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61094-1 as well as
the following apply.
3.1
electrostatic actuator
device for determination of microphone frequency response comprising an electrically
conductive stiff plate placed near the microphone diaphragm such that a time-varying voltage,
applied between the plate and the diaphragm, produces an electrostatic force that simulates a
sound pressure uniformly distributed over the surface of the diaphragm
3.2
electrostatic actuator response of a microphone
microphone output as a function of frequency measured using a specified design of electro-
static actuator driven by a voltage that is of uniform amplitude with frequency, relative to the
output at a specified frequency
NOTE Electrostatic actuator response is expressed in decibels (dB).
3.3
acoustic radiation impedance
acoustic impedance loading the microphone diaphragm on its outer surface
–
3
NOTE 1 Acoustic radiation impedance is expressed in pascal-second per cubic meter (Pa⋅s⋅ m ).
NOTE 2 The radiation impedance depends on the presence and design of the actuator.
4 Reference environmental conditions
The reference environmental conditions are:
temperature 23,0 °C
static pressure 101,325 kPa
relative humidity 50 %
5 Principle of electrostatic actuator operation
5.1 General
In practice, measurements of sound are made in many different environments where different
types of sound fields exist. The sensitivity and frequency response of measurement
microphones depend on the type of sound field, so ideally the microphone should be
calibrated using a similar type of field to that which exists on the measurement site. The
various types of sound fields are generally approximated by three idealized fields: free field,
diffuse-field and pressure-field.
However, the establishment of such idealized sound fields, which are suitable for calibration
of measurement microphones over the frequency ranges of interest is technically difficult and
requires costly acoustical laboratory facilities. Therefore, the electrostatic actuator method is
used for determining a relative frequency response of measurement microphones. This
method, which accounts for the type of sound field by using specific predetermined
corrections, requires no such facilities.

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61094-6  IEC:2004 – 15 –
At higher frequencies, the free-field sensitivity of a microphone is determined by the
behaviour of its diaphragm and the sound diffraction and reflection caused by the microphone.
The effect of the diaphragm behaviour, which may cause significant differences in the relative
frequency responses between individual microphones of the same model, requires specific
determination. This frequency response determination is performed using the electrostatic
actuator method.
The effect of the diffraction and reflection depends on the type of sound field and on the
shape and dimensions of the microphone. As these parameters are essentially the same for
all microphones of the same model, the influence of diffraction and reflection does not differ
significantly between individual microphones of the same model.
Therefore, corrections for specific types of sound field may be determined once for a model of
microphone and subsequently applied to the electrostatic actuator response of any
microphones of that model.
Free-field and pressure-field corrections are calculated by determining the respective
frequency responses of one or more microphones of the same model by using acoustical
calibration methods, for example, those in IEC 61094-2 and IEC 61094-3, and by subtracting
the respective electrostatic actuator responses.
In principle, the electrostatic actuator calibration method may be used from very low to very
high frequencies. However, the actuator excites the microphone diaphragm only and not the
static pressure equalisation vent, which is generally exposed to sound when measurements
are made in a free-field. The actuator excitation corresponds to that of a pressure-field and
thus cannot be used for determination of the lower limiting frequency under free-field
conditions. Free-field response determinations by electrostatic actuator should only be made
at frequencies which are at least 10 times greater than the lower limiting frequency derived
from the time constant of the venting system of the microphone. At low frequencies, a small
perforation in the microphone diaphragm will exhibit different effects in the actuator response
and in the acoustic responses in a pressure field or a free field. At high frequencies, the
degree to which the actuator excitation approximates that of a pressure field depends on the
relation between the acoustic impedance of the microphone diaphragm and the acoustic
radiation impedance of the microphone diaphragm with the actuator in place. This relation is
described in 5.3, while 9.3 describes some practical consequences for the determination of
the environmental characteristics of a microphone.
5.2 Electrostatic pressure
The rigid and electrically conductive plate of the actuator is placed close to and parallel to the
microphone diaphragm, see Figure 1. It forms an electrical capacitor together with the
microphone diaphragm, which shall also be electrically conductive. When a voltage is applied
between the capacitor plates, the actuator produces a force F distributed over the diaphragm
surface; see Equation (1) below.
The corresponding electrostatically produced pressure p is defined by Equation (2). Edge
act
effects are neglected. The ratio between the effective actuator area and the active diaphragm
area gives a constant, which is generally less than unity because the actuator is perforated
for acoustic reasons.

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61094-6  IEC:2004 – 17 –

3
4
+
d
F
U
2
-
1
IEC  1507/04

Key

1 Microphone housing
2 Microphone diaphragm. Area S
dia

3 Electrostatic actuator. Area S
act

4 Holes
Figure 1 – Principle of microphone and electrostatic actuator
ε S
gas act
2
F =− U (1)
2
2d
ε
F
gas
2
p==− aU (2)
act
2
S
2d
dia
where
F is the electrostatic force produced on diaphragm (a pushing or pulling force is
considered to be positive or negative respectively), in newtons (N);
p is the electrostatically produced pressure on the diaphragm, in pascals (Pa);

act
ε is the dielectric constant of gas in space between actuator and diaphragm, in
gas
–12
farads per meter (F/m) (in air: ε = 8,85 × 10 F/m);
gas
d is the effective distance between actuator and diaphragm, in meters (m);
2
S is the active diaphragm area, in square meters (m );
dia
S is the effective surface area of actuator above the active diaphragm area, in
act
2
square meters (m );
S
act
a = is the ratio between effective actuator area and active diaphragm area;
S
dia
U is the voltage applied between actuator and microphone diaphragm, in volts (V).
Actuators are generally operated with a d.c voltage and a superimposed sinusoidal a.c
voltage. Equation (3) describes the instantaneous electrostatic pressure on the diaphragm for
this mode of operation.

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61094-6  IEC:2004 – 19 –
ε a 2
gas
p()tU=− +u 2sin()ωt (3)
()0
2
2d
The Equations (4), (5) and (6) describe the resulting electrostatic pressure components, which
include the desired equivalent sound pressure p at the fundamental frequency and two non-
nd
desired components, a 2 harmonic pressure p and a static pressure p .
d stat
ε a
gas
p = Uu (4)
0
2
d
ε a
gas
2
p = u (5)
d
2
22 d
ε a
gas
22
p =− Uu+ (6)
( )
stat 0
2
2d
where
p(t) is the equivalent instantaneous sound pressure, in pascals (Pa);
p is the r.m.s. value of the sound pressure at the fundamental frequency, in pascals (Pa);
nd
p is the r.m.s. value of the sound pressure at the 2 harmonic frequency, in pascals (Pa);

d
p is the static pressure, in pascals (Pa);

stat
t is the time, in seconds (s);
U is the d.c. voltage applied between actuator and microphone diaphragm, in volts (V);
0
u is the r.m.s. value of the a.c. voltage applied between actuator and microphone
diaphragm, in volts (V);
ω is the angular frequency, in radians per second (rad/s).
The equation below defines the fraction of distortion, i.e. the ratio between the magnitudes of
the second harmonic and the fundamental frequency components:
u

D = 100 % (7)
2 2U
0
Examples of the design of electrostatic actuators are given in Annex A and an example of a
measurement set-up in Annex B.
NOTE 1 Although Equation (4) describes the absolute value of the equivalent sound pressure produced on the
microphone diaphragm, the actuator method is usually only used for measurement of relative frequency response.
The method might be used for determination of absolute microphone sensitivity but the resulting uncertainty would
generally be too large for most applications. Relatively large uncertainty is associated with the determination of the
distance d and with the ratio of areas a.
NOTE 2 Actuators may also be operated with a.c. voltage only. Equations (3), (4), (5) and (6) are also valid for
this mode of operation (U = 0). It should be noticed that the frequency of the electrostatically produced equivalent
0
pressure becomes twice the frequency of the supplied electrical signal, and that any variation of voltage input level
causes a change in this equivalent sound pressure level that is twice as large.
NOTE 3 The influence of the distortion of the excitation signal, see equation (7), on the microphone output signal
depends on the frequency response of the microphone. This influence can be eliminated by using a selective
measurement technique to measure the fundamental frequency component only.

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61094-6  IEC:2004 – 21 –
5.3 Electrostatic actuator response
The electrostatic actuator method uses an electrostatically produced pressure to excite the
microphone diaphragm. A constant electrostatic pressure may in practice be produced on a
microphone diaphragm over a wide frequency range by keeping the driving a.c. voltage
constant while its frequency is varied.
The movement of the microphone diaphragm caused by the electrostatic excitation produces
a sound pressure on the diaphragm in addition to the electrostatic pre
...